1. Field of the Invention
The present invention relates to a method and an apparatus for nuclear magnetic resonance imaging using a shim coil which adjusts a static magnetic field to improve the homogeneity of the static magnetic field and, more particularly, to a mechanism for determining an amount of current to be supplied to the shim coil in such a nuclear magnetic resonance apparatus.
2. Description of the Background Art
As is well-known, in a nuclear magnetic resonance imaging apparatus (referred hereafter as MRI apparatus) nuclear magnetic resonance signals (referred hereafter as NMR signals) are obtained from a region of interest in a body to be examined by irradiating the region with radio frequency excitation pulses (referred hereafter as RF pulses) under the presence of a static magnetic field surrounding the region, and image reconstructions or the spectrum analyses are carried out on a basis of obtained NMR signals.
In a case of image reconstruction, a high-resolution image is obtainable when the homogeneity of the static magnetic field in the region of interest is within the range of 1 to several ppm. However, the homogeneity of the static magnetic field can easily be disturbed beyond this range by the presence of the body to be examined.
To cope with this situation, the adjustment of the homogeneity of the static magnetic field by superposing an extra magnetic field produced by a shim coil placed in the static magnetic field with various currents flowing therethrough an amount of which determines the strength of the extra magnetic field is commonly employed. A passive use of the shim coil in which the induced dipole field appearing as the shim coil is placed in the static magnetic field is utilized for adjusting the static magnetic field can also be found in practice. The adjustment of the current flowing through shim coil has an advantage of being capable of changing the strength of the extra magnetic field and thereby adjusting the homogeneity of the static magnetic field in an active manner.
In such an adjustment of the current flowing a shim coil, it is convenient to control the amount of the current flowing in the shim coil in accordance with the NMR signals being collected, so that in a conventional MRI apparatus equipped with such a current flowing shim coil the amount of the current is determined in accordance with the NMR signals collected from the particular species of atoms selected for the purpose of the specific type of examination to be carried out, regardless of the species selected.
As a result, it has been difficult to achieve an accurate adjustment when the NMR signals are weak which occurs, for instance, in a case in which the selected species is phosphorus (.sup.31 p).
On the other hand, even in a case such as one in which the selected species is hydrogen (.sup.1 H) light of the for which the NMR signals are strong, when the body to be examined is a human being the NMR signals of the hydrogen contains contributions from both water and fat which possess different resonant frequencies, so that accurate adjustment was also difficult to achieve.
Thus, in a conventional MRI apparatus it has often been difficult to achieve accurate adjustment of the homogeneity of the static magnetic field.